Assignment Detail

PART 1 Isomerization of Olefins in Fluidised Bed Reactors
1. BACKGROUND
Negative environmental consequences of fossil fuels and concerns about petroleum supplies have spurred the search for second-generation biofuels. To be a viable alternative, these biofuels must have environmental benefits, be economically competitive, and have the same (or even better) quality as the fuels they are to replace. For example, hydrocarbon olefins available from pyrolysis of waste wood can be catalytically isomerized to yield olefins having more desirable physical or chemical properties or more similar characteristics to current motor fuels. Olefin isomerization may be used to supply highoctane motor fuel for which the lowerboiling olefins are especially useful, or to supply branched-chain  hydrocarbons for subsequent hydrogenation to yield high-octane paraffins for sustainable aviation fuels (SAF).

The purpose of this investigation is to test a newly developed catalyst for the isomerization of light olefins in a bubbling fluidised bed reactor operated at 500 °C and atmospheric pressure. The fluid fed to the reactor is a gaseous mixture containing 20 vol% of 1-hexene (C6H12) in inert nitrogen (N2, 80 vol%). In the reactor, 1-hexene converts into 2-hexene according to the reaction:
The chosen catalyst particles are spherical, with particle density 2700 kg m-3 and in the size range of 350-1000 μm, as indicated in the particle size distribution in Table

Q.1. The solid fraction of the bed at rest (no flow of gas) is equal to 0.55. Table Q.1: Particle Size Distribution (of a 120 g sample) Size range (μm) Mass (g)
350-400 1
400-450 2.5
450-500 4.1
500-550 5.1
550-600 5.3
600-650 8.5
650-700 15.1
700-750 25.6
750-800 20.5
800-850 17.6
850-900 6.1
900-950 5.4
950-1000 3.2
4
2. REACTOR CHARACTERISTICS
a) Determine the Sauter mean diameter of the catalysts particles and classify them according to Geldart
classification. [3]
b) During operation, the reactor is operated at 4 times the minimum fluidization velocity. Demonstrate
that, at this velocity, the reactor behaves like a bubbling fluidized bed, and estimate the fraction of
catalyst particles that are carried upwards with the product gas leaving the reactor. [7]
3. REACTOR OPERATION
c) Using the two-phase theory, we can model the isomerization reactor as shown in Figure Q.3.
The meaning of the symbols adopted and the values of some of them are reported in Table Q.3. Explain
using entirely your own words what the two-phase theory states and discuss the assumptions on which
the two-phase theory of Toomey & Johnstone is based. [3]
Figure Q.3: Schematic block diagram of the bubbling fluidized bed combustor.
d) Draw the concentration profiles of 1-hexane in the radial direction within the catalyst particle for: i) a
very active catalyst and ii) a nearly inactive catalyst. Comment on the shape of the concentration
profiles. [4]
e) When very active catalysts are used, the catalyst pellets are often prepared by depositing the expansive
catalytic material in a thin layer on the outside of the pellet surface. Explain the reason for this. [3]
f) The number of moles of 1-hexene that convert in one catalyst particle per unit time can be calculated using the relationship: , where the subscript A refers to the reactant, 1-hexene. Prove that: Report all the steps in your derivation. [4]
g) Calculate the external mass transfer resistance related to the flow of 1-hexene from the bulk of the
emulsion phase to the outer surface of the particles. [4]
h) Assuming that the internal mass transfer resistance is negligible compared to the external one, calculate the value of , i.e. of the concentration of 1-hexene in the emulsion phase of the fluidized bed.
[10]
i) Calculate the value of , i.e. of the concentration of 1-hexene at the outlet of the fluidizedbed reactor.
[2]
4. DESIGN & OPERATIONAL CHANGES
j) Due to the mild exothermicity of the reaction, a heat exchange system would be required to maintain
isothermal conditions during prolonged operation. Estimate the heat transfer coefficient on a vertical
exchanger surface immersed in the reactor during normal operation. [5]
k) A number of electrically heated tubes of constant temperature 150 °C are available to be used as heat
exchanging surface. Each tube has 10mm external diameter and 2m length. Estimate the number of
tubes that would be needed to operate the isomerization reactor at constant temperature of 500 °C. [10]
l) Repeat calculations for questions g), h), i), j), and k) using 4 different superficial velocities of your
choice, at which the reactor would still operate at bubbling regime. Discuss the above results and explain what the effect of gas superficial velocity is on the operation of the isomerization reactor, both from a hydrodynamic and chemical reaction perspectives. Finally, state if you would run the reactor at a specific velocity, and explain why you would do so. [35]

PART 2 Design of Industrial Crystallization List and explain the main factors that determine the crystal size distribution (CSD) from a continuous wellmixed crystallizer. Show, with the aid of an information-flow diagram, how a change in one of such factors affects the others.  Finally, propose the main design and operation principles for two-different crystallisers containing aqueous solutions of potassium nitrate (KNO3) and sodium chloride (NaCl), respectively. Clearly describe the differences in the two cases, and justify your choices. [10]